Up to now it was possible to change the pattern for all images, but
there are use-cases where different variants might want different names.
For example there could be one main variant that should only have
product name in the ISO filename, but addons should still be marked with
variant name.
JIRA: COMPOSE-3041
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>